The proposed research will evaluate in autochthonous murine mammary cancers a treatment program which has been shown to be successful in transplanted solid tumors. The treatment consists of the injection of an oncolytic enterovirus (Jo) to infect the tumors coupled with an appropriate administration of cyclophosphamide. The studies will be done in A/Strong mice. The dynamics of in vivo in tumor virus growth will be studied as a function of time and cyclophosphamide dosage in both autochthonous and transplanted tumors. The course of viral induced pathological events in (Jo) enterovirus infected mammary tumor cells will be described using electron microscopic techniques.